Methods of fabricating silicon carbide devices having smooth channels

ABSTRACT

Methods of forming silicon carbide power devices are provided. An n −  silicon carbide layer is provided on a silicon carbide substrate. A p-type silicon carbide well region is provided on the n −  silicon carbide layer. A buried region of p +  silicon carbide is provided on the p-type silicon carbide well region. An n +  region of silicon carbide is provided on the buried region of p +  silicon carbide. A channel region of the power device is adjacent the buried region of p +  silicon carbide and the n +  region of silicon carbide. An n −  region is provided on the channel region and a portion of the n −  region is removed from the channel region so that a portion of the n −  region remains on the channel region to provide a reduction in a surface roughness of the channel region.

STATEMENT OF GOVERNMENT INTEREST

The present invention was developed with Government support undercontract number FA8650-04-2-2410 awarded by the United States Air Force.The Government has certain rights in this invention.

FIELD OF THE INVENTION

This invention relates to methods of fabricating power devices and theresulting devices and, more particularly, to silicon carbide powerdevices and methods of fabricating silicon carbide power devices.

BACKGROUND OF THE INVENTION

Power devices are widely used to carry large currents and support highvoltages. Modern power devices are generally fabricated frommonocrystalline silicon semiconductor material. One widely used powerdevice is the power Metal Oxide Semiconductor Field Effect Transistor(MOSFET). In a power MOSFET, a control signal is supplied to a gateelectrode that is separated from the semiconductor surface by anintervening insulator, which may be, but is not limited to, silicondioxide. Current conduction occurs via transport of majority carriers,without the presence of minority carrier injection that is used inbipolar transistor operation. Power MOSFETs can provide an excellentsafe operating area, and can be paralleled in a unit cell structure.

As is well known to those having skill in the art, power MOSFETs mayinclude a lateral structure or a vertical structure. In a lateralstructure, the drain, gate and source terminals are on the same surfaceof a substrate. In contrast, in a vertical structure, the source anddrain are on opposite surfaces of the substrate.

One widely used silicon power MOSFET is the double diffused MOSFET(DMOSFET), which is fabricated using a double-diffusion process. Inthese devices, a p-base region and an n⁺ source region are diffusedthrough a common opening in a mask. The p-base region is driven indeeper than the n⁺ source. The difference in the lateral diffusionbetween the p-base and n⁺ source regions forms a surface channel region.An overview of power MOSFETs including DMOSFETs may be found in thetextbook entitled “Power Semiconductor Devices” by B. J. Baliga,published by PWS Publishing Company, 1996, and specifically in Chapter7, entitled “Power MOSFET”, the disclosure of which is herebyincorporated herein by reference.

Development efforts in power devices have also included investigation ofthe use of silicon carbide (SiC) devices for power devices. Siliconcarbide has a wide bandgap, a lower dielectric constant, a highbreakdown field strength, a high thermal conductivity, and a highsaturation electron drift velocity compared to silicon. Thesecharacteristics may allow silicon carbide power devices to operate athigher temperatures, higher power levels and with lower specificon-resistance than conventional silicon-based power devices. Atheoretical analysis of the superiority of silicon carbide devices oversilicon devices is found in a publication by Bhatnagar et al. entitled“Comparison of 6H—SiC, 3C—SiC and Si for Power Devices”, IEEETransactions on Electron Devices, Vol. 40, 1993, pp. 645-655. A powerMOSFET fabricated in silicon carbide is described in U.S. Pat. No.5,506,421 to Palmour entitled “Power MOSFET in Silicon Carbide” andassigned to the assignee of the present invention. Silicon Carbide powerdevices are also described in U.S. Pat. No. 6,107,142 to Suvorov et al.,entitled Self-Aligned Methods of Fabricating Silicon Carbide PowerDevices by Implantation and Lateral Diffusion and U.S. Pat. No.6,100,169 to Suvorov et. al., entitled Methods of Fabricating SiliconCarbide Power Devices by Controlled Annealing, both of which areassigned to the assignee of the present invention.

Notwithstanding these potential advantages, it may be difficult tofabricate power devices including power MOSFETs in silicon carbide. Forexample, as described above, the double-diffused MOSFET (DMOSFET) isgenerally fabricated in silicon using a double diffusion process whereinthe p-base region is driven in deeper than the n⁺ source. Unfortunately,in silicon carbide, the diffusion coefficients of conventional p andn-type dopants are small compared to silicon, so that it may bedifficult to obtain the required depths of the p-base and n⁺ sourceregions using acceptable diffusion times and temperatures. Ionimplantation may also be used to implant the p-base and the n⁺ source.See, for example, “High-Voltage Double-Implanted Power MOSFET's in6H—SiC” by Shenoy et al., IEEE Electron Device Letters, Vol. 18, No. 3,March 1997, pp. 93-95. However, it may be difficult to control the depthand lateral extent of ion implanted regions. Moreover, the need to forma surface channel surrounding the source region may require the use oftwo separate implantation masks. It may then be difficult to align thep-base and the source regions to one another, thereby potentiallyimpacting the device performance. Furthermore, performing a seriesimplants and implant activation anneals may result in step bunchedsurfaces across the channel of the MOSFET, which may diminish deviceperformance in terms of, for example, increased on resistance andreduced reliability.

SUMMARY OF THE INVENTION

Some embodiments of the present invention provide methods of formingsilicon carbide power devices. An n⁻ silicon carbide layer is providedon a silicon carbide substrate. A p-type silicon carbide well region isprovided on the n⁻ silicon carbide layer. A buried region of p⁺ siliconcarbide is provided on the p-type silicon carbide well region. An n⁺region of silicon carbide is provided on the buried region of p⁺ siliconcarbide. A channel region of the power device is adjacent the buriedregion of p⁺ silicon carbide and the n⁺ region of silicon carbide. An n⁻region is provided on the channel region and a portion of the n⁻ regionis removed from the channel region so that a portion of the n⁻ regionremains on the channel region to provide a reduction in a surfaceroughness of the channel region.

In further embodiments of the present invention, a portion of the n⁻region may be removed using a chemical mechanical polishing (CMP)process that removes a portion of the n⁻ region from the channel region.The CMP process may remove all but from about 1000 to about 5000 Å ofthe n⁻ region. A portion of the n⁻ region that is from about 2.0 toabout 3.0 times a depth of the surface roughness of the channel regionmay be removed. In certain embodiments of the present invention about1500 Å of the n⁻ region remains on the channel region after the CMPprocess.

In still further embodiments of the present invention, the reduction inthe surface roughness may be a reduction in a root mean square (RMS)surface roughness of from at least about 28 Å to less than about 1.0 Å.A sacrificial oxide layer having a thickness of from about 100 to about1000 Å may be formed on the remaining portion of the n⁻ region on thechannel region and removed. The RMS surface roughness may be furtherreduced by the formation and removal of the sacrificial oxide layer fromless than about 1.0 Å to about 0.70 Å.

In some embodiments of the present invention, the n⁻ region may be an n⁻epitaxial layer that is grown on the channel region to a predeterminedthickness such that a portion of the n⁻ epitaxial layer remains on thechannel region after removal of a portion of the n⁻ epitaxial layer. Incertain embodiments of the present invention, the predeterminedthickness of the n⁻ epitaxial layer may be from about 1500 Å to about5000 Å. The CMP process may be followed by selectively etching the n⁻region such that the n⁻ region is removed from the n⁺ region.

In further embodiments of the present invention, the p-type siliconcarbide well region may be formed by implanting p-type dopants in the n⁻silicon carbide layer. The buried region of p⁺ silicon carbide may beformed by implanting p-type dopants in the p-type silicon carbide wellregion. The n⁺ region of silicon carbide may be formed by implantingn-type dopants in the p-type silicon carbide well region on the buriedregion of p⁺ silicon carbide. The implanted dopants may be activated byexposing the implanted dopants to a temperature of greater than about1600° C.

In still further embodiments of the present invention, the p-typesilicon carbide well region may be a p-type epitaxial layer on the n⁻silicon carbide layer. The buried regions of p⁺ silicon carbide may beformed by implanting p-type silicon carbide dopants on the p-typesilicon carbide well region. The n⁺ regions of silicon carbide may beformed by implanting n-type silicon carbide dopants in the p-typesilicon carbide well region on the buried region of p⁺ silicon carbide.

In some embodiments of the present invention, an n-type region ofsilicon carbide may be formed in the p-type silicon carbide well regionadjacent the channel region. The channel region may be defined betweenthe buried region of p⁺ silicon carbide and the n-type region of siliconcarbide. The n-type region of silicon carbide may be a Junction FieldEffect Transistor (JFET) region of the silicon carbide power device.

In further embodiments of the present invention, an n-type region ofsilicon carbide may be formed in the p-type silicon carbide well regionadjacent the channel region. The channel region may be defined betweenthe buried region of p⁺ silicon carbide and the n-type region of siliconcarbide. The n⁻ region may only remain on the channel region.

In still further embodiments of the present invention, the substrate maybe an n⁻ substrate, which serves as a drift region of the siliconcarbide power device. An n⁺ drain region may be provided on thesubstrate opposite the n⁻ silicon carbide layer. The n⁺ drain region maybe an implanted region or an epitaxial region. In certain embodiments ofthe present invention, the silicon carbide power device may be a MetalOxide Semiconductor Field Effect Transistor (MOSFET).

Some embodiments of the present invention provide methods of formingsilicon carbide power devices. A silicon carbide epitaxial region isgrown on a channel region of the power device. A portion of the siliconcarbide epitaxial region is mechanically removed such that a remainingportion of the silicon carbide epitaxial region provides a reduction ina surface roughness of the channel region.

In still further embodiments of the present invention, the reduction inthe surface roughness may be a reduction in the RMS surface roughness offrom at least about 28 Å to less than about 1.0 Å. In certainembodiments of the present invention, a sacrificial oxide layer may beformed on the silicon carbide epitaxial region and the sacrificial oxidelayer may be removed to provide a further reduction in the surfaceroughness of the channel region. The RMS surface roughness may befurther reduced by the formation and removal of the sacrificial oxidelayer from less than about 1.0 Å to about 0.70 Å.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross section illustrating power MOSFETs according to someembodiments of the present invention.

FIGS. 2A through 2J are cross sections illustrating processing steps inthe fabrication of the power MOSFETs of FIG. 1 according to someembodiments of the present invention.

FIGS. 3A through 3C are images of channels of MOSFETS fabricated usingmethods according to some embodiments of the present invention.

FIG. 4 is a chart illustrating a comparison of Drain Voltage (V) vs.Drain Current (A) for conventional devices and devices fabricated usingmethods according to some embodiments of the present invention.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

The present invention now will be described more fully hereinafter withreference to the accompanying drawings, in which embodiments of theinvention are shown. This invention may, however, be embodied in manydifferent forms and should not be construed as limited to theembodiments set forth herein. Rather, these embodiments are provided sothat this disclosure will be thorough and complete, and will fullyconvey the scope of the invention to those skilled in the art. In thedrawings, the size and relative sizes of layers and regions may beexaggerated for clarity. It will be understood that when an element orlayer is referred to as being “on”, “connected to” or “coupled to”another element or layer, it can be directly on, connected or coupled tothe other element or layer or intervening elements or layers may bepresent. In contrast, when an element is referred to as being “directlyon,” “directly connected to” or “directly coupled to” another element orlayer, there are no intervening elements or layers present. Like numbersrefer to like elements throughout. As used herein, the term “and/or”includes any and all combinations of one or more of the associatedlisted items.

It will be understood that, although the terms first, second, etc. maybe used herein to describe various elements, components, regions, layersand/or sections, these elements, components, regions, layers and/orsections should not be limited by these terms. These terms are only usedto distinguish one element, component, region, layer or section fromanother element, component, region, layer or section. Thus, a firstelement, component, region, layer or section discussed below could betermed a second element, component, region, layer or section withoutdeparting from the teachings of the present invention.

Furthermore, relative terms, such as “lower” or “bottom” and “upper” or“top,” may be used herein to describe one element's relationship toanother elements as illustrated in the Figures. It will be understoodthat relative terms are intended to encompass different orientations ofthe device in addition to the orientation depicted in the Figures. Forexample, if the device in the Figures is turned over, elements describedas being on the “lower” side of other elements would then be oriented on“upper” sides of the other elements. The exemplary term “lower”, cantherefore, encompass both an orientation of “lower” and “upper,”depending on the particular orientation of the figure. Similarly, if thedevice in one of the figures is turned over, elements described as“below” or “beneath” other elements would then be oriented “above” theother elements. The exemplary terms “below” or “beneath” can, therefore,encompass both an orientation of above and below.

Embodiments of the present invention are described herein with referenceto cross-section illustrations that are schematic illustrations ofidealized embodiments of the present invention. As such, variations fromthe shapes of the illustrations as a result, for example, ofmanufacturing techniques and/or tolerances, are to be expected. Thus,embodiments of the present invention should not be construed as limitedto the particular shapes of regions illustrated herein but are toinclude deviations in shapes that result, for example, frommanufacturing. For example, an implanted region illustrated as arectangle will, typically, have rounded or curved features and/or agradient of implant concentration at its edges rather than a binarychange from implanted to non-implanted region. Likewise, a buried regionformed by implantation may result in some implantation in the regionbetween the buried region and the surface through which the implantationtakes place. Thus, the regions illustrated in the figures are schematicin nature and their shapes are not intended to illustrate the preciseshape of a region of a device and are not intended to limit the scope ofthe present invention.

Embodiments of the present invention are described with reference to aparticular polarity conductivity type for various layers/regions.However, as will be appreciated by those of skill in the art, thepolarity of the regions/layers may be inverted to provide an oppositepolarity device.

Some embodiments of the present invention prevention provide power MetalOxide Semiconductor Field Effect Transistors (MOSFETs) and/or otherpower devices that may include doped regions of silicon carbide. As isknown to those of skill in the art, doped regions of silicon carbide maybe formed through epitaxial growth and/or through implantation. Forexample, a p-type region of silicon carbide may be formed throughepitaxial growth in the presence of a p-type dopant or throughimplantation of p-type dopants in an undoped, p-type or n-type epitaxiallayer. The structure that results from epitaxial growth differs fromthat that results from implantation. Thus, the terms “epitaxial region”and “implanted region” structurally distinguish differing regions ofsilicon carbide and may be used herein as a recitation of structuralcharacteristics of the regions of silicon carbide and/or as recitationsof methods of forming such regions of silicon carbide.

While embodiments of the present invention are described with referenceto MOSFETs, embodiments of the present invention may be used in otherdevices, such as laterally-diffused MOSFETs (LDMOSFETs), Insulated GateBipolar Transistors (IGBTs) or other such MOS based devices as well ashigh voltage devices, such as Schottky and PiN diodes without departingfrom the scope of the present invention.

Surface morphology is a concern in the development of commercial siliconcarbide (SiC) power devices, such as SiC power MOSFETs. The series ofimplantation steps and implant activation anneals may result instep-bunched surfaces across a channel of the device. The steps createdon the surface of the device may be several hundred angstroms, which maycause increased on resistances and reduced reliability of the device.Methods according to some embodiments of the present invention mayprovide channel regions having reduced surface roughness so as toimprove overall device performance. In particular, methods offabricating power devices according to some embodiments of the presentinvention may provide an additional n⁻ region on the channel region. Inparticular, an n⁻ region may be formed on the channel region of thedevice, a portion of which is removed, such that a remaining portion ofthe n⁻ region on the channel region provides a reduced surface roughnessof the channel region. Thus, at least a portion of the step-bunchedsurface may be removed from the channel region to provide a smootherchannel region, i.e., a channel region with reduced surface roughness.In some embodiments, the removal process is performed using a chemicalmechanical polishing process that may remove a portion of, or from about1200 Å to about 1400 Å, of the n⁻ region from the channel region. Theremaining portion of the n⁻ region may provide a reduction in a rootmean square (RMS) surface roughness of the channel region of from aboutat least 28 Å to less than about 1.0 Å as will be discussed furtherbelow with respect to FIGS. 1 through 4.

Referring now to FIG. 1, a cross section illustrating embodiments of avertical power MOSFET according to some embodiments of the presentinvention will be discussed. It will be understood by those having skillin the art that the vertical silicon carbide MOSFETs are generallyreplicated in a unit cell. Such unit cells are illustrated between lines100 a and 100 c or lines 100 b and 100 c. For ease of illustration, atwo unit cell MOSFET will be described, however, as will be appreciatedby those of skill in the art, additional unit cells may be incorporatedinto a MOSFET along one direction or along two, generally orthogonaldirections, without departing from the scope of the present invention.

As illustrated in FIG. 1, a MOSFET according to some embodiments of thepresent invention includes an n⁺ monocrystalline silicon carbidesubstrate 10, for example, 4H silicon carbide. An n⁻ silicon carbidelayer 12 is provided on a first face 10A of the substrate 10. As usedherein, “p⁺” or “n⁺” refer to regions that are defined by higher carrierconcentrations than are present in adjacent or other regions of the sameor another layer or substrate. Similarly, “p⁻” or “n⁻” refer to regionsthat are defined by lower carrier concentrations than are present inadjacent or other regions of the same or another layer or substrate.

In some embodiments of the present invention, an n⁻ substrate couldserve as an n⁻ drift layer of the device. In these embodiments of thepresent invention, an n⁺ drain region may be provided on a second face10B of the n⁻ substrate. The n⁺ drain region may be provided by ionimplantation or epitaxial growth without departing from the scope of thepresent invention.

A p-type silicon carbide region 14 is provided on the n⁻ silicon carbidelayer 12 and may provide a p-well region of the MOSFET. It will beunderstood that the p-well region may be an epitaxial region or animplanted region without departing from the scope of the presentinvention.

A buried region 18 of p⁺ silicon carbide is provided in the p-wellregion 14 beneath an n⁺ region 20 of silicon carbide that is alsoprovided in the p-type region 14. The n⁺ region 20 may provide a sourceregion of the device. Adjacent and spaced apart from the n⁺ sourceregion 20 is an n-type silicon carbide region 21 that extends throughthe p-well region 14 to the n⁻ silicon carbide layer 12. It will beunderstood that in embodiments of the present invention where the p-wellregion 14 is an epitaxial layer, the n-type silicon carbide region 21may be termed a Junction Field Effect Transistor (JFET) region of thedevice.

The n-type silicon carbide region 21 may provide a portion of an n-typechannel region 29. A surface of the n-type silicon carbide region 21 mayinclude unwanted steps of up to several hundred angstroms each, whichmay cause increased on resistances and reduced reliability of thedevice. Furthermore, a surface of the p-type region 31 between then-type silicon carbide region 21 and the buried region 18 of p⁺ siliconcarbide may also included these unwanted steps. Thus, as illustrated inFIG. 1, an n⁻ region 19′ according to some embodiments of the presentinvention may be provided on the n-type silicon carbide region 21 andthe p-type region 31, which may form a channel region 29 of the device.In particular, an n-type epitaxial layer may be regrown on the n-typesilicon carbide region 21, the p-type region 31 and other surfaces ofthe structure and selectively removed such that a portion of the n⁻region 19′ remains on the n-type silicon carbide region 21 and thep-type region 31 of the device, but is removed from the n⁺ sourceregions 20 as will be discussed further below with respect to FIGS. 2Athrough 2J.

Regrowth is discussed in detail in commonly assigned United StatesPatent Application Publication No. US 2004/0211980 entitled SiliconCarbide Power Devices With Self-Aligned Source And Well Regions andMethods Of Fabricating Same, filed on Apr. 24, 2003; United StatesPatent Application Publication No. US 2004/0119076 entitled VerticalJFET Limited Silicon Carbide Power Metal-Oxide Semiconductor FieldEffect Transistors and Methods of Fabricating Vertical JFET LimitedSilicon Carbide Metal-Oxide Semiconductor Field Effect Transistors,filed on Oct. 30, 2003; and United States Patent Application PublicationNo. US 2002/0038891 entitled Silicon Carbide Power Metal-OxideSemiconductor Field Effect Transistors Having a Shorting Channel andMethods of Fabricating Silicon Carbide Metal-Oxide Semiconductor FieldEffect Transistors Having a Shorting Channel, filed Jun. 24, 2001, thedisclosures of which are hereby incorporated herein by reference as ifset forth in their entirety.

The presence of the n⁻ region 19′ on the n-type silicon carbide region21 and the p-type region 31 may provide a reduction in a surfaceroughness of the channel region 29 (a combination of the n-type siliconcarbide region 21, the p-type region 31 and the remaining portion of then− region 19′). In other words, the n⁻ region 19′ may fill in some ofthe peaks and valleys on the surface of the n-type silicon carbideregion 21 and the p-type region 31 to provide a smoother interface 35between the channel region 29 and the gate insulating layer 30(discussed below). According to some embodiments of the presentinvention, the channel region 29 may have a root mean square (RMS)surface roughness of less than about 1.0 Å. Thus, according to someembodiments of the present invention, a reduction in the RMS surfaceroughness of from at least about 28 Å to less than about 1.0 Å may beobtained, which will be discussed further below with respect to FIGS. 2Athrough 3C. According to some embodiments of the present invention, anoptional sacrificial oxide layer (not shown) may be provided on asurface of the channel region 29 and subsequently removed. Thesacrificial oxide layer may have a thickness of from about 100 to about1000 Å, for example 500 Å. The formation and removal of the sacrificialoxide layer may further reduce the RMS surface roughness of the surfaceof the channel region 29 to about 0.70 Å.

A gate insulating layer 30 of a suitable dielectric material, such assilicon dioxide (SiO₂), extends over the channel region 29 and to the n⁺source regions 20. A gate contact 26 is provided on the gate layeropposite the channel region 29, including the n-type silicon carbideregion 21, the p-type region 31 and the n− region 19′. Source contact 24is provided between the n⁺ source regions 20 and a drain contact 28 isprovided on a second face 10B of the substrate 10 opposite the p-typeregion 14.

It will be understood that although embodiments of the present inventionare discussed with respect to the MOSFET structure illustrated in FIG.1, embodiments of the present invention are not limited to thisconfiguration. For example, in some embodiments of the presentinvention, the MOSFET may include a hybrid well structure as describedin commonly assigned U.S. patent application Ser. No. 10/873,394, filedJun. 22, 2004, entitled Silicon Carbide Devices With Hybrid Well RegionsAnd Methods Of Fabricating Silicon Carbide Devices With Hybrid WellRegions, the disclosure of which is incorporated herein by reference asif set forth fully herein.

Referring now to FIGS. 2A through 2J, processing steps in thefabrication of power devices, for example, MOSFETs, according to someembodiments of the present invention will be discussed. As illustratedin FIG. 2A, an n⁻ epitaxial layer 12 is formed on a n⁺ silicon carbidesubstrate 10, for example, a 4H silicon carbide substrate. The n⁻ layer12 may have a thickness of from about 5 to about 200 μm and a doping toprovide a carrier concentration of from about 1×10¹⁴ cm⁻³ to about1×10¹⁷ cm⁻³. In particular embodiments of the present invention, then-layer 12 is about 12 μm thick and is doped to provide a carrierconcentration of about 5×10¹⁵ cm⁻³.

A p-type layer 14 is provided on the n⁻ epitaxial layer 12. As discussedabove, the p-type layer 14 may provide a p-well region of the device.The p-type layer 14 may be an epitxial region or an implanted regionwithout departing from the scope of the present invention. Inparticular, in embodiments of the present invention where the p-typelayer 14 is an implanted region, p-type silicon carbide dopants may beimplanted in the n⁻ layer 12 to provide the p-type layer 14. Inembodiments of the present invention where the p-type layer 14 is anepitaxial layer, the p-type layer 14 may be grown on the n⁻ layer 12.The p-type layer 14 may have a thickness of from about 0.5 to about 3 μmand a doping to provide a carrier concentration of from about 2×10¹⁶cm⁻³ to about 5×10¹⁷ cm⁻³. In particular embodiments of the presentinvention, the p-type layer 14 is about 0.5 μm thick and is doped toprovide a carrier concentration of about 1×10¹⁶ cm⁻³. As furtherillustrated in FIG. 2A, a mask layer 100 is formed on the p-type layer14 and patterned to form openings corresponding to the source regions 20and the buried p-type regions 18 of the device.

As illustrated in FIGS. 2B and 2C, the buried p-type regions 18 and thesource regions 20 may be formed by patterning a mask layer 100 havingopenings corresponding to the location of the source regions 20. Theburied p-type regions 18 are formed by ion implantation of p-typedopants using the patterned mask 100. The buried p-type regions 18 mayextend to and, in some embodiments, into the drift region, for example,n⁻ layer 12. The p-type dopant may be, for example, aluminum or othersuitable p-type dopant. In certain embodiments of the present invention,the p-type region 18 has a thickness of less than about the thickness towhich the region 21 may be formed, for example, from about 0.2 μm toabout 1 μm. In particular embodiments of the present invention, theburied p-type regions 18 extend from a depth of about 0.2 μm to a depthof about 0.7 μm from the surface of the p-type layer 14. Furthermore,the p-type region 18 may be doped to provide a carrier concentration offrom about 1×10¹⁷ cm⁻³ to about 1×10¹⁸ cm⁻³.

As illustrated in FIG. 2C, the source regions 20 are formed byimplanting n-type dopants into the p-type layer 14 utilizing thepatterned mask 100. The n-type dopant used for n-type implants may be,for example, nitrogen and/or phosphorus, however, other n-type dopantsmay also be used. The n-type source regions 20 may extend a distance offrom about 0.2 μm to about 0.3 μm into the p-type layer. The n-typesource regions 20 may be doped to provide a carrier concentrationsufficient to allow for formation of a good ohmic contact. As usedherein the term “ohmic contact” refers to contacts where an impedanceassociated therewith is substantially given by the relationship ofImpedance=V/I, where V is a voltage across the contact and I is thecurrent, at substantially all expected operating frequencies (i.e., theimpedance associated with the ohmic contact is substantially the same atall operating frequencies) and currents. In particular embodiments ofthe present invention, the n-type source regions extend to a depth ofabout 0.2 μm into the p-type layer and are doped to provide a carrierconcentration of at least from about 1×10¹⁹ cm⁻³ to about 1×10²⁰ cm⁻³.

While embodiments of the present invention are described herein usingthe same windows in the patterned mask 100 for implantation of both thep-type regions 18 and the source regions 20, in some embodiments of thepresent invention different size windows may be used to compensate forstraggle in the implantation.

As illustrated in FIGS. 2D through 2E, the mask 100 may be removed andan n-type silicon carbide region 21 of the device may be provided in thep-well region 14. The n-type silicon carbide region 21 may be animplanted region or an epitaxial region without departing from the scopeof the present invention. In embodiments of the present invention wherethe n-type silicon carbide region 21 is an implanted region, a secondmask layer 120 may be patterned to have opening corresponding to then-type silicon carbide region 21 of the device as illustrated in FIG.2D. It will be understood that in embodiments of the present inventionwhere the p-well region 14 is an epitaxial layer, the n-type siliconcarbide region 21 may be termed a Junction Field Effect Transistor(JFET) region of the device.

As illustrated in FIG. 2E, the n-type silicon carbide region 21 may beformed by implanting n-type dopants in the p-type layer 14 utilizing thepatterned mask 120 so as to form the n-type silicon carbide region 21that extends through the p-type layer 14 to the drift region, forexample, the n-type epitaxial layer 12. Any suitable n-type dopants maybe used. In some embodiments of the present invention, nitrogen is usedas the n-type dopant.

When the device is turned-on, the n-type region 21 can provide a pathfrom the MOS channel to the lightly doped drift region (12), allowingelectrons to flow from source regions to drain regions. In theoff-state, this n-channel region may be depleted of electrons from thereverse biased pn junction, which is formed on both sides of the channelregion. The pn-junctions on both sides of the channel region may shieldthe MOS region from high electric fields at off state, which may resultin a higher device reliability compared to trench devices, such asUMOSFETs.

FIG. 2F illustrates the formation of an optional cap layer 140 of adeposited oxide and/or other passivating material. The cap layer 140 mayhave a thickness of from about 0.01 μm to about 1 μm. In any event,whether a cap layer 140 is utilized or not, the device may be exposed toa high temperature anneal ranging from about 900° C. to about 1800° C.and in some embodiments, about 1600° C. for several minutes, such asfive minutes, so as to activate the n-type and p-type implants.

As illustrated in FIG. 2G, the cap layer 140 may be removed and an n⁻silicon carbide epitaxial region 19 may be formed on the channel region29 of the device. A depth of the n⁻ silicon carbide epitaxial region 19may be controlled so that only a portion of the n⁻ region 19 may beremoved from the channel region 29 in a subsequent processing step. Insome embodiments of the present invention, the n⁻ silicon carbide regionmay have a thickness of from about 1500 Å to about 6000 Å and a carrierconcentration of about 5×10¹⁵ cm⁻³.

As illustrated in FIG. 2H, a portion of the n⁻ region 19 is removed. Theportion of the n⁻ region 19′ that remains on the n-type silicon carbideregion 21 and the p-type region 31 may define the channel region 29(FIG. 2I) of the device. The remaining portion of the n⁻ region 19′ mayhave a thickness of from about 1000 to about 5000 Å. In some embodimentsof the present invention, the remaining portion of the n⁻ region 19′ mayhave a thickness of from about 1500 Å. The presence of the remainingportion of the n⁻ region 19′ on the channel region 29 may provide areduction in a surface roughness of the channel region 29 at aninterface 35 between the n-region 19′ and the gate insulator 30, whichwill be discussed further below with respect to FIG. 3.

It will be understood that that the channel region 29 may be defined bythe remaining portion of the n⁻ region 19′ and the portion of the p-well31 between the n-type silicon carbide region 21 and the buried p-typeregions 18 and the source regions 20. Accordingly, the n⁻ region 19′ mayonly remain on the portions of the p-well 31 between the n-type siliconcarbide region 21 and the buried p-type regions 18 and the sourceregions 20 and not on the n-type silicon carbide region 21 withoutdeparting from the scope of the present invention.

In some embodiments of the present invention, a chemical mechanicalpolishing (CMP) process may be performed that removes a portion of then⁻ region 19 from a surface of the channel region 29. In certainembodiments of the present invention, the CMP process may remove aportion of the n− region 19 that is from about 2.0 to 3.0 times a depthof the surface roughness of the channel region 29. It will be understoodthat any conventional CMP process may be used as the CMP processdiscussed herein without departing from the scope of the presentinvention. However, the process discussed herein does not remove theentire n⁻ region 19. Typically, implanted regions have a thickness of afew thousand angstroms and conventional CMP processes remove severalmicrons of film during the CMP process to obtain the desired result.However, as will be understood by those having skill in the art, removalof several microns of film from the structure of FIG. 2G would removeall of the implanted region rendering the device non-functional.

Accordingly, some embodiments of the present invention use a CMP processthat removes a portion of the n⁻ region 19, leaving a portion of the n⁻region 19′ remaining on the channel region 29. For example, in someembodiments of the present invention, the CMP process may be performedfor less than about 3 hours, for example, from about half an hour toabout an hour. In other words, after the CMP process from about 1000 toabout 5000 Å of the n⁻ region 19 may remain on the channel region 29 ofthe device. Thus, the remaining portion of the n⁻ region may be fromabout 1000 to about 5000 Å. The remaining portion of the n⁻ region 19′on the n-type silicon carbide region 21 and the p-well region 31 becomepart of the channel region 29, i.e., the remaining portion of the n−region 19′ may fill in the peaks and valleys on a surface of the n-typesilicon carbide region 21 and the p-well region 31. As furtherillustrated in FIG. 2H, a mask 130 may be provided on the remainingportion of the n⁻ region 19′ and the n⁻ region 19′ may be patternedaccording to the mask 130, removing portions of the n⁻ region 19′ on thesource regions 20 as illustrated in FIG. 2I. As discussed above, in someembodiments of the present invention the n− region 19′ may only remainon the p-well region 31 between the n-type silicon carbide region 21 andthe source regions 20 and the buried p-type regions 18.

As discussed above, providing the n⁻ region 19′ on the n-type region 21and the p-well region 31 may provide a reduction in the surfaceroughness between the n− region 19′ and the gate insulator 30. Forexample, as illustrated in FIG. 3A, the surface of the n-type siliconcarbide region 21 and the p-well region 31 may have many peaks andvalleys after the anneal that activates the implanted regions. Thesepeaks and valleys (several 100 angstroms) may cause suboptimalperformance of the device. After the forming of the n⁻ region 19 on then-type silicon carbide region 21 and the p-well region 31 and theremoval of a portion of the n⁻ region from the n-type silicon carbideregion 21 and the p-well region 31 according to some embodiments of thepresent invention, many of the peaks and valleys may be removed by thepresence of the remaining portion of the n⁻ region 19′ as illustrated inFIG. 3B. In particular, the root mean square (RMS) surface roughness ofthe channel region 29 may be reduced from at least about 28 Å in FIG. 3Ato less than about 1.0 Å in FIG. 3B according to some embodiments of thepresent invention. Furthermore, in some embodiments of the presentinvention, an optional sacrificial oxide layer (not shown) may be formedon a surface of the channel region 29 and removed. The formation andremoval of the sacrificial oxide layer may further reduce the RMSsurface roughness to about 0.70 Å as illustrated in FIG. 3C.

As illustrated in FIG. 2I, a layer 30 of insulating material may bedeposited on the device so as to provide a gate insulating material. Insome embodiments of the present invention, the insulating material maybe formed as described in U.S. Pat. No. 6,610,366, entitled Method ofN₂O annealing an oxide layer on a silicon carbide layer, U.S. Pat. No.6,767,843, entitled Method of N₂O growth of an oxide layer on a siliconcarbide layer and/or United States Patent Application Publication No.US2002/0102358A1, entitled Method of fabricating an oxide layer on asilicon carbide layer utilizing an anneal in a hydrogen environment, thedisclosures of which are incorporated herein as if set forth in theirentirety. In certain embodiments of the present invention, theinsulating material may be formed as described in Improved InversionChannel Mobility for 4H—SiC MOSFETs Following High Temperature Annealsin Nitric Oxide to Chung et al., the disclosure of which is incorporatedherein by reference as if set forth in its entirety. As furtherillustrated in FIG. 2I, a gate contact 26 may be formed by forming ametal contact on the gate insulating material 30. Suitable gate contactmaterials include, but are not limited to, aluminum, polysilicon andmolybdenum. Furthermore, as will be appreciated by those of skill in theart, a multi-layered gate contact may also be used.

As is illustrated in FIG. 2J, contact holes may be formed in theinsulating material 30′ and ohmic contacts may be formed in the contactholes to provide source contacts 24. Similarly, an ohmic contact 28 maybe formed on the substrate 10. Suitable ohmic contact materials include,but are not limited to, nickel, titanium alloy, and aluminum. Thedeposited metal contacts may then be annealed at a high temperatureranging from about 500° C. to about 1200° C. An overlayer metal may alsobe deposited on the contacts, for example, to facilitate connection tothe device.

It will be understood by those having skill in the art that the order ofsteps in FIGS. 2A through 2J may be changed. Thus, for example, then-type silicon carbide region 21 may be formed before forming the n⁺regions 20 without departing from the scope of the present invention.Similarly, the buried p⁺ regions 18 may be formed either before or afterformation of the n⁺ regions 20 or the n-type silicon carbide region 21.Similarly, the contacts 24 may be formed by, for example, depositing andpatterning a metal layer, and then the insulating layer 30′ provided andopenings in the insulating layer formed to the contacts 24. Accordingly,the present invention should not be construed as limited to the exactsequence of operations described herein but is intended to encompassother sequences of fabrication that will become apparent to those ofskill in the art in light of the present disclosure.

Referring now to FIG. 4, a chart illustrating a comparison of DrainVoltage (V) vs. Drain Current (A) for conventional devices and devicesfabricated using methods according to some embodiments of the presentinvention will be discussed. As illustrated in FIG. 4, the draincurrents of MOSFETs fabricated according to some embodiments of thepresent invention are higher than conventional MOSFETS at relative drainvoltages.

In the drawings and specification, there have been disclosed typicalpreferred embodiments of the invention and, although specific terms areemployed, they are used in a generic and descriptive sense only and notfor purposes of limitation, the scope of the invention being set forthin the following claims.

1. A method of forming a silicon carbide power device, comprising:forming an n⁻ silicon carbide layer on a silicon carbide substrate;forming a p-type silicon carbide well region on the n⁻ silicon carbidelayer; forming a buried region of p⁺ silicon carbide in the p-typesilicon carbide well region; forming an n⁺ region of silicon carbide onthe buried region of p⁺ silicon carbide, a channel region of the powerdevice being adjacent the buried region of p⁺ and n⁺ region of siliconcarbide; forming an n⁻ region on the channel region; and removing aportion of the n⁻ region from the channel region so that a portion ofthe n⁻ region remains on the channel region to provide a reduction in asurface roughness of the channel region.
 2. The method of claim 1,wherein removing comprises performing a chemical mechanical polishing(CMP) process that removes a portion of the n⁻ if region from thechannel region.
 3. The method of claim 2, wherein the CMP processremoves all but from about 1000 to about 5000 Å of the n⁻ f region fromthe channel region.
 4. The method of claim 2, wherein removing furthercomprises removing a portion of the n⁻ if region that is from about 2.0to about 3.0 times a depth of the surface roughness of the channelregion.
 5. The method of claim 4, wherein about 1500 Å of the n⁻ regionremains on the channel region after the CMP process.
 6. The method ofclaim 1, wherein the reduction in the surface roughness is a reductionin a root mean square (RMS) surface roughness of from at least about 28Å to less than about 1.0 Å.
 7. The method of claim 6, wherein removingis followed by: forming a sacrificial oxide layer on the remainingportion of the n-region on the channel region, the sacrificial layerhaving a thickness of from about 100 to about 1000 Å; and removing thesacrificial oxide layer, wherein the RMS surface roughness is furtherreduced by the formation and removal of the sacrificial oxide layer fromless than about 1.0 Å to about 0.70 Å.
 8. The method of claim 1, whereinforming the n³¹ region comprises growing an n⁻ epitaxial layer on thechannel region to a predetermined thickness such that a portion of then⁻ epitaxial layer remains on the channel region after removal of theportion of the n⁻ epitaxial layer.
 9. The method of claim 8, wherein thepredetermined thickness of the n⁻ epitaxial layer is from about 1500 Åto about 6000 Å.
 10. The method of claim 2, wherein performing the CMPprocess is followed by selectively etching the n⁻ region such that then⁻ region is removed from the n³⁰ region.
 11. The method of claim 1:wherein forming the p-type silicon carbide well region comprisesimplanting p-type dopants in the n⁻ if silicon carbide layer; whereinforming the buried region of p⁺ silicon carbide comprises implantingp-type dopants in the p-type silicon carbide well region; and whereinforming the n⁺ region of silicon carbide comprises implanting n-typedopants in the p-type silicon carbide well region on the buried regionof p³⁰ silicon carbide.
 12. The method of claim 11, further comprisingactivating the implanted dopants by exposing the implanted dopants to atemperature of greater than about 1600° C.
 13. The method of claim 2:wherein forming the p-type silicon carbide well region comprises forminga p-type epitaxial layer on the n⁻ silicon carbide layer; whereinforming the buried region of p⁺ silicon carbide comprises implantingp-type silicon carbide dopants in the p-type silicon carbide wellregion; and wherein forming the n³⁰ region of silicon carbide comprisesimplanting n-type silicon carbide dopants in the p-type silicon carbidewell region on the first and second p⁺ regions of silicon carbide. 14.The method of claim 13, further comprising forming an n-type region ofsilicon carbide in the p-type silicon carbide well region adjacent thechannel region, wherein the channel region is defined between the buriedregion of p⁺ silicon carbide and the n-type region of silicon carbideand wherein the n-type region of silicon carbide is a Junction FieldEffect Transistor (JFET) region of the silicon carbide power device. 15.The method of claim 1, further comprising forming an n-type region ofsilicon carbide in the p-type silicon carbide well region adjacent thechannel region, wherein the channel region is defined between the buriedregion of p⁺ silicon carbide and the n-type region of silicon carbideand wherein the n⁻ region only remains on the channel region.
 16. Themethod of claim 1, wherein the substrate comprises n⁻ substrate whichserves as a drift region of the silicon carbide power device, the methodfurther comprising forming an n⁺ drain region on the substrate oppositethe n⁻ silicon carbide layer.
 17. The method of claim 16, whereinforming the n⁺ drain region comprises at least one of implanting the n⁺drain region in the n⁻ substrate or growing the n⁺ drain region on then⁻ substrate.
 18. The method of claim 1, wherein the silicon carbidepower device comprises a Metal Oxide Semiconductor Field EffectTransistor (MOSFET).
 19. A method of forming a silicon carbide powerdevice, comprising: growing a silicon carbide epitaxial region on achannel region of the power device; mechanically removing a portion ofthe silicon carbide epitaxial region such that a remaining portion ofthe silicon carbide epitaxial region on the channel region provides areduction in a surface roughness of the channel region; and thenselectively etching the silicon carbide epitaxial region to expose anunderlying region while a portion of the silicon carbide epitaxialregion remains on the channel region of the power device.
 20. The methodof claim 19, wherein the reduction in the surface roughness is areduction in a root mean square (RMS) surface roughness of from at leastabout 28 Å to less than about 1.0 Å.
 21. The method of claim 20, furthercomprising: forming a sacrificial oxide layer on the remaining portionof the silicon carbide epitaxial region, the sacrificial oxide layerhaving a thickness of from about 100 to about 1000 Å; and removing thesacrificial oxide layer to provide a further reduction in the surfaceroughness of the channel region.
 22. The method of claim 21, wherein theRMS surface roughness is further reduced by the formation and removal ofthe sacrificial oxide layer from less than about 1.0 Å to about 0.70 Å.23. The method of claim 19, wherein mechanically removing comprisesperforming a chemical mechanical polishing (CMP) process that removes aportion of the silicon carbide epitaxial region from the channel region.24. The method of claim 23, wherein a remaining portion of the siliconcarbide epitaxial region after the CMP process has a thickness of fromabout 1000 to about 5000 Å.
 25. The method of claim 19, whereinmechanically removing further comprises mechanically removing a portionof the silicon carbide epitaxial region that is from about 2 to about 3times a depth of the surface roughness of the channel region.
 26. Themethod of claim 25, wherein a remaining portion of the silicon carbideepitaxial region has a thickness of from about 1500 Å.
 27. The method ofclaim 19, wherein the silicon carbide epitaxial region has a thicknessfrom about 1500 Åto about 6000 Å.
 28. The method of claim 19, furthercomprising: forming an n⁻ silicon carbide layer on a silicon carbidesubstrate; forming a p-type silicon carbide well region on the n⁻silicon carbide layer; forming first and second spaced apart buriedregions of p⁺ silicon carbide in the p-type silicon carbide well region;and forming first and second spaced apart n⁺ regions of silicon carbideon the first and second p⁺ regions of silicon carbide, respectively,such that the channel region of the power device is defined between thefirst p⁺ and n⁺ regions of silicon carbide and the second p⁺ and n⁺regions of silicon carbide.
 29. The method of claim 28: wherein formingthe p-type silicon carbide well region comprises implanting p-typedopants in the n⁻ silicon carbide layer; wherein forming the first andsecond spaced apart buried regions of p⁺ silicon carbide comprisesimplanting p-type dopants in the p-type silicon carbide well region; andwherein forming the first and second spaced apart n⁺ regions of siliconcarbide comprises implanting n-type dopants in the p-type siliconcarbide well region on the first and second p⁺ regions of siliconcarbide.
 30. The method of claim 29, further comprising activating theimplanted dopants by exposing the implanted dopants to a temperature ofgreater than about 1600 ° C.